|
The Wave That Shook The World
Chat to oceanographer Dr Simon Boxall after the Equinox programme on
Monday 24 January at 10pm.
Dr Duncan L Copp
January 2005
The surface of the planet we live on may seem unchanging, yet
geological forces powered by the interior heat of Earth, left over
from its formation, constantly stress and strain the crust deep
beneath our feet. Tsunamis are caused by sudden cataclysmic
upheavals in this crust. It was one such upheaval that caused the
disastrous waves that struck the Indian Ocean on Boxing Day.
Earth's crust is not a continuous shell, but is broken into
approximately eight major sections, called tectonic plates. Great
stresses and strains occur at the boundaries of these plates, where
they jostle past each other. Here the crust is either being torn
apart or is sinking below another section. Periodically, these
pent-up stresses release themselves in the sudden shudder of an
earthquake or the violent eruption of a volcano. Plate boundaries
can be found on the continents and also under the oceans. Should a
particularly strong shift occur in the crust along a plate boundary
under the ocean, the result will be a powerful earthquake, often
followed by a tsunami.
A tsunami is a direct result of the displacement (either up or
down) of the ocean crust at an underwater plate boundary, which in
turn displaces the water above. The Boxing Day tsunami occurred
after the displacement of crust along a 1000-kilometre stretch of
the India Plate where the crust is sinking under the Burma
Plate.
Tsunamis can also occur when massive landslides slip into the
sea, triggered by a volcanic eruption, earthquake, or both. During
the eruption of Krakatoa in 1883, large sections of the island
collapsed into the Sunda Straights near Java. This displaced a huge
amount of water, resulting in a series of tsunamis, the greatest of
which was 40 metres high.
Beach waves
Normal waves we see on the beach are formed by the passage of
wind over the ocean or lake. The wind pushes and piles up the
surface of the water, forming a 'swell'. The size of a swell depends
on two main factors: the strength of the wind and the distance the
wind has blown over the open water, known as the fetch. Typically, a
long fetch and strong winds makes for big swells. The huge rollers
loved by surfers in Hawaii and California owe their existence to
powerful tropical storms way out in the Pacific.
Swells remain relatively constant while out at sea, but when
driven in to shore their characteristics change. As the depth of
water decreases, the swell slows down and grows taller, forming a
wave. The height a wave can reach, called the amplitude, depends
mainly on the distance between two successive wave crests, called
the 'wavelength' – the larger the wavelength the greater the
amplitude of the wave. Typically, a series of big Californian
rollers may have wavelengths of 150 metres or so and amplitudes of a
few metres. Eventually, the wave grows so high it becomes unstable,
arching over before breaking onto the beach.
The speed at which a wave travels depends on water depth, and how
deep the wave's energy extends vertically through the water – the
greater the water depth, the faster it can travel. Normally, waves
formed by wind don't extend much beyond a few tens of metres below
the water. This depth and speed relationship explains why waves slow
down when they come into shore; as the water depth decreases, so
does the wave's velocity.
Tsunami waves
While the formation of tsunamis is different from normal beach
waves, their physical characteristics are the same, only of much
greater proportions. The effective depth of a tsunami wave is far
greater than a beach wave, since the displacement of crust affects
the water above it vertically through the whole depth of the ocean,
from the seabed to the sea surface. This can be as much as 4000
metres in the Pacific. As depth affects wave speed, tsunamis can
travel at ferocious speeds in the open ocean – up to 830kmh – as
fast as a jumbo jet.
After the initial displacement of the crust, a series of tsunamis
will ripple outwards in a similar way to the ripples on a pond from
a pebble. Their wavelength, however, is much greater than swells
caused by wind. There can be hundreds of kilometres between the
crests of tsunamis, compared to the tens of metres for wind-produced
swells. While out at sea, a tsunami may only be a metre or so high
and hardly noticeable to ocean-going ships. But the height radically
changes as the tsunami moves towards the land.
The majority of continents are surrounded by a flat shallow shelf
typically a 100 metres or so underwater. As the tsunami approaches
this 'continental shelf', its speed slows and height builds – in the
same way that wind-generated swells do when they meet the shallow
water of the seashore. It's here that tsunamis grow into huge walls
of water. Owing to their enormous wavelengths, tsunamis can grow
many tens of metres in height when they reach the shallow
waters.
Why tsunamis are so destructive
When on the move, water is incredibly powerful. Many people get
into difficulty while swimming in a river or the sea simply because
they underestimate the energy stored in moving water. The energy
stored up in a wave is proportional to the square of its height.
This simple relationship explains why tsunamis are so
destructive.
As an example, a typical fairweather 1-metre high wave delivers
as much energy as 10 one-bar electric heaters for every metre of
shoreline that it hits. But a 20-metre high tsunami, for example,
will deliver 20 x 20 = 400 times as much energy – that's 4000
one-bar heaters. This explains why waves are one of the most
destructive forces known to man.
The long wavelengths of tsunamis mean that the crests are
typically spread out over greater distances compared to a normal
beach wave. The result is that tsunami waves last much longer when
they break on the shore.
Added to all this is the weight of water: a cubic metre weighs
one tonne. It's the continuous fast-moving and sudden flood of water
which is so damaging. The sheer volume of water from multiple
tsunami waves travelling at high-speed means little can stand in the
way, even with relatively small tsunamis just a few metres high.
The coastline can influence the destructive outcome of a tsunami.
The distance a tsunami moves inland is known as the run-up. With
wide, flat expanses of beach, the wave energy dissipates relatively
quickly. A band of vegetation running along the shore, such as a
mangrove swamp, can also greatly reduce the run-up of a tsunami.
However, the opposite can occur with inlets and bays. They can act
as funnels for tsunamis, restricting and focusing their destructive
force.
Predicting tsunamis
What can be done to protect against tsunamis? Unfortunately,
predicting events which lead to their formation (earthquakes and
volcanic eruptions) is still scientifically challenging. However,
following the tsunami that wreaked havoc on the shores of the
Hawaiian islands on 1 April 1946, a modest tsunami warning system
was installed in the Pacific, mainly to alert US military bases.
This has since grown into a much more comprehensive network, known
as the Pacific Tsunami Warning System (PTWS). Twenty-four seismic
stations dotted throughout the Pacific monitor earthquake activity,
measuring their magnitudes which can be used to help calculate the
risk of tsunamis along the Pacific coastline.
Unfortunately, no such system exists for the Indian Ocean. Here,
powerful earthquakes responsible for causing tsunamis are much
rarer. However, the sad fact is that had such a system been in
operation on 26 December 2004, coastal districts would have had up
to three hours to be evacuated, possibly saving tens of thousands of
lives.
While tsunamis are devastating, they are relatively simple to
avoid, given adequate evacuation time. The best way to escape the
incoming water once a warning has been issued is to head for high
ground – this need only be a few tens of metres in elevation. Even
better is to try and head inland as far as possible. It's very rare
that the run-up of a tsunami reaches for more than a few
kilometres.
Without any early warning, the first indication that a tsunami is
imminent is often the sea retreating from the shoreline, revealing
an unnaturally large expanse of seabed. The water is being drawn up
by the tsunami as the crest develops when the wave moves into
shallower water. On seeing this, you must act quickly; you may only
have a few minutes before the wall of water strikes.
You can read more about the recent tsunami and the global
economic isues it has raised on the Channel 4 site After the
Tsunami.
Find out more
Channel 4 is not responsible for the content of third party
sites
Websites
2004 Indian Ocean Earthquake
http://en.wikipedia.org/wiki/2004_Indian_
Ocean_earthquake#Signs_and_warnings
Excellent
and up-to-date information from Wikipedia. Explains how an
earthquake caused the tsunami devastation, with an interactive map
and a tsunami timeline of key events in the Indian Ocean. Also
covers the need to have an early warning system in place, as is
currently the case in the Pacific.
Asian disaster – How to help
http://news.bbc.co.uk/1/hi/world
/asia-pacific/4131881.stm
BBC
site shows which countries were affected by the disaster, and offers
a comprehensive list of agencies asking for help.
Oxfam demands trade relief for countries affected by
tsunami
www.oxfam.org.uk/press/releases/
asiaquake_trade_120105.htm
Oxfam
argue that EU Trade Commissioner Peter Mandelson should act
immediately by removing all tariffs on textiles and clothing imports
to the EU from affected countries.
Plate tectonics and people
http://pubs.usgs.gov/publications/text/
tectonics.html#anchor24562816
Good
overview of how plate tectonics affect Earth and the people that
inhabit it.
The South-East Asia earthquake and Tsunami Blog
http://tsunamihelp.blogspot.com/
Blogs often
have more up-to-date news than the usual media outlets so log on
here to find out what's being done and what's still left to do. This
site has tons of links and information.
Tsunami early warning 'next year'
http://news.bbc.co.uk/2/hi/south_asia/4168917.stm
Unesco
is set to give the go-ahead for an Indian Ocean early warning
centre. This would involve placing a number of deep water measuring
devices on the ocean floor, which would relay wave movements to
surface buoys and then to a satellite.
Tsunami Earthquake Appeal – Disasters Emergency
Committee
http://www.dec.org.uk/
This is the main British
Appeal, which represents key UK agencies like Oxfam, British Red
Cross, Save the Children, Action Aid and others. Donate online, via
text or phone. Text 'donate' to 83321 and £1.50 will be donated to
the appeal (no VAT or charges will be made). Phone 08700 60 60
900.
Tsunami in Asia – UK citizens missing
www.fco.gov.uk/servlet/Front?pagename=
OpenMarket/Xcelerate/ShowPage
&c=Page&cid=1022686957237
The
Foreign Office has up-to-date information on those British citizens
known to be dead or missing. There is an emergency number for
concerned friends and relatives and travel advice on those countries
affected.
The Tsunami Page
www.drgeorgepc.com/
Comprehensive pages from a
tsunami expert, with FAQs, diagrams, articles and publications and a
wealth of information on tsunamis, volcanoes, earthquakes,
hurricanes and more.
Books
top
^ |